1 This might be caused by mud balls or accumulations of dirt
and sand held together by organic growths. The common correction is to apply 2
ounces of calcium hypochlorite per square foot of filter surface and soak the
medium as described above. Maintenance of adequate backwash rates and
application of pool disinfectant chemicals ahead of the filters will probably prevent
this difficulty from recurring.
2 Some success has been reported with the use of an acid bath
for correcting clogged filters and under-drains. Two pounds of sodium bisulfate per
square foot of filter area is added, and the filters are permitted to soak as described
above. Thorough backwashing is critical after this treatment to prevent damage to the
metal parts of the system.
3 Cementation of sand grains due to mineral compounds normally
in the water or used in water treatment may prevent correction by these treatments. In
this case the sand might have to be replaced.
4 Sodium hexametaphosphate may be fed into water to prevent
the undesirable effects of excessive hardness. Dosing in the range of 5 ppm is
generally accepted. One suggested technique for batch feeding is to add 10 pounds of
the compound for each 250,000 gallons of pool capacity at the start and 2 pounds for
each 250,000 gallons every other week.
c. High Rate Sand Filters.
(1) Operation. These filters contain only sand and should be operated 24
hours per day at the manufacturer's suggested rate (12 to 20 gpm per sq. ft.). When the
filter differential pressure reaches 10 to 15 psi, dirt will have penetrated the sand to a
depth of 6 to 10 inches. The high rate filters often operate at pressures up to 30 or 40
psi. At a 10 to 15 psi increase, backwashing is necessary for only a 2 to 4 minutes
period at the same flow rate as the filtering process.
(a) Backwashing with water that is not clean may clog the openings in
the tank collection manifold. This clogging will cause uneven filter and backwash flow.
The sand must be removed and the openings physically cleaned if the clogging does
not correct itself during the next filter run. Draining the pool by use of the backwash
cycle will cause clogging of the precision openings. Failure to clean the openings will
cause increased velocity, which will eat away the remaining openings. This leads to
destruction of the manifold laterals.
(b) Automatic and continuous bleeding of air from the filter tank is